Adipocytes and myocytes contain intracellular GLUT4-containing vesicles which fuse to the plasma membrane upon insulin stimulation. The resulting increase in plasma membrane GLUT4 is responsible for the 10 to 20-fold increase in glucose transport in the insulin-stimulated state. GLUT4-containing vesicle movement from the cytoplasm to fusion with the plasma membrane, a process termed translocation, is believed to be abnormal in both insulin resistance and non-insulin dependent diabetes mellitus (NIDDM) (B. Kahn, J. Clin. Invest., 89, 1367-1374, (1992)).
The normal molecular mechanisms of insulin-stimulated translocation of GLUT4-containing vesicles to the plasma membrane and the concomitant increase in glucose uptake by the affected cells are still largely unknown. Proposed mechanisms include both GLUT4-containing vesicle docking and fusion with the plasma membrane.
The GLUT4-containing vesicles are only a minor fraction of the total intracellular vesicle population within adipocytes (James et al, J. Biol. Chem., 262, 11817-11824, (1987)). These GLUT4 vesicles can be isolated from the low density microsomal fraction using immunoaffinity methods with antibodies to the cytoplasmic-oriented C-terminus of GLUT4 (James, et al, J. Biol. Chem., 262, 11817-11824, (1987); Thoidis, et al, J. Biol. Chem., 268, 11691-11696, (1993)). GLUT4-enriched vesicles have been shown to contain several synaptic vesicle-like proteins including vesicle associated membrane protein (VAMP) (Cain et al, J. Biol. Chem., 267, 11681-11684, (1992)), secretory carrier associated membrane protein (SCAMP) (Laurie et al, J. Biol. Chem., 268, 19110-19117, (1993)), and certain ras analog proteins originally identified in brain (Rab proteins) (Cormont, et al, J. Biol. Chem., 268, 19491-19497, (1993)).
Presently the pharmacologic therapy for NIDDM may not target the insulin resistance which is the key pathophysiological abnormality of the disease. The existing therapies are 1) diet and exercise; 2) sulfonylureas, which stimulate insulin secretion; 3) .alpha.-glucosidase inhibitors, which inhibit the enzymatic digestion of complex carbohydrates and thereby slow the postprandial absorption of glucose; 4) metformin, whose mechanism likely includes improvement of hepatic insulin sensitivity, and 5) insulin injections.
A therapeutic agent which targets insulin resistance and improves insulin sensitivity would have significant advantages over the therapies listed above. Such a therapeutic agent is the subject of the present invention. It is an objective of this invention to increase insulin sensitivity by modulating the aminopeptidase associated with GLUT4 vesicles.